1. Field of the Invention
The present invention relates to a light-emitting element using electroluminescence. Further, the present invention relates to a light-emitting device and an electronic device using the light-emitting element. The present invention relates to a lighting device using the light-emitting device.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence have been extensively conducted. In the basic structure of such a light-emitting element, a layer including a light-emitting substance is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting substance.
Since this type of light-emitting element is a self-luminous type, it has advantages over a liquid crystal display in that visibility of a pixel is high and that no backlight is needed. Therefore, light-emitting elements are thought to be suitable as display elements. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response speed is very high.
Further, since this type of a light-emitting element can be formed to have a film shape, surface light emission can be easily obtained. This feature is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Therefore, such light-emitting elements also have a high utility value as surface light source that can be applied to lighting apparatuses or the like.
Light-Emitting elements using electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance.
When an organic compound is used as a light-emitting substance, by application of a voltage to a light-emitting element, electrons and holes are injected into a layer including the light-emitting organic compound from a pair of electrodes, whereby a current flows. Then, carriers (i.e., electrons and holes) recombine to place the light-emitting organic compound into an excited state. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element.
Note that as types of an excited state of an organic compound, a singlet excited state and a triplet excited state are known. It is thought that light emission can be obtained through either of the excited states.
In an attempt to improve element characteristics, there are many problems which depend on a material used, and in order to solve these problems, improvement of element structure, development of a material, and the like have been carried out.
For example, to apply a light-emitting element to an active matrix type display device, usually, the light-emitting element is formed over an element substrate provided with a transistor that controls light emission and the like. A problem with such a structure has been that a bottom emission structure where light emitted from a light-emitting element is extracted to the outside through an element substrate provided with a transistor and the like could decrease aperture ratio due to a wiring, the transistor, or the like.
In order to solve this problem, a structure by which emitted light is extracted from the side opposite an element substrate provided with a transistor and the like (a top emission structure) is disposed (see Patent Document 1, for example). Use of a top emission structure contributes to improvement of the aperture ratio, resulting in an increase in emission efficiency.